Lithium Niobate MZ modulators are commonly used in modern optical transmission equipment to generate very high quality transmit signals. The high quality transmit signals can propagate long distances having high performance margin to counteract transmission impairments such as optical noise and dispersion. To a first order these modulators are insensitive to a wide range of input powers and wavelengths. However these types of modulator are prone to aging and temperature drift effects. Closed loop control schemes need to be implemented to successfully implement these components into real-life transmission equipment products. There are a few established techniques for controlling MZ modulators that are applicable to standard WDM transmitters, for example as disclosed in US Patent publication number US2006/0210210, assigned to Azea Networks ltd.
Typically amplitude modulated transmitters are set up and controlled for maximum extinction ratio with the logic ‘1’ input signal corresponding to the maximum throughput of the modulator and the logic ‘0’ input signal corresponding to the minimum throughput of the modulator. To achieve maximum optical extinction ratio the electrical drive signal amplitude should create a peak-to-peak phase difference of 180 degrees between the two light levels in the two combining arms of the MZ. A drive amplitude that creates a 180 degree peak-to-peak phase difference is commonly referred to as a ‘Vpi’ drive signal amplitude. A DC bias voltage is also required such that the absolute phase difference for the two input logic level voltages occur at 0 and 180 degree phase difference points, corresponding to the maximum and minimum light levels. The optimum DC bias voltage is commonly referred to as the quadrature bias point, as shown in FIG. 1.
For Lithium Niobate MZ modulators the optimum DC bias voltage can drift significantly over time and temperature, therefore a bias control loop is required to maintain optimum performance, as shown in FIG. 2. The drive signal amplitude requirement (e.g. Vpi) for the modulator does not change significantly, however the drive signal amplifier and associated electronics can age, so a control loop is usually implemented to maintain a Vpi drive signal.
There are several known modulator bias control schemes. An example would be one that periodically compares the modulator output optical power to an optimised reference value to determine an error signal for bias correction. This approach requires a calibrated look up table for the entire input power and wavelength range, and is prone to the problem of aging effects over time.
An alternative approach is to use a dither control signal with the error signal determined from a relative optical power difference. This technique has the advantage of working over a wide input power and wavelength range without the need for a complex calibration procedure. For bias control, a small perturbing dither signal is applied the driver gain. The dither signal can be generated using analogue control electronics or digital microprocessor control loops. The driver gain variation causes a small step change to the drive signal amplitude that in turn causes a small optical power change at the modulator output. The optical power change is used as the error signal to correct the bias voltage. For a MZ modulator, the optical power change (or error signal) conveniently tends to zero at the optimum bias position, as shown in FIG. 2.
Alternatively, the driver amplitude can be optimised with a similar routine. Here the bias voltage can be dithered instead of gain, and the optical power change monitored in the same way, to create an error signal for the driver gain. It is also possible to run both bias and gain dither optimisation routines sequentially, with the optimum bias or gain value being found regardless of the other bias/gain state. The dither routine is designed to operate at a faster rate than any temperature and aging effects, therefore the optimum bias and gain position can be maintained over life.
Both direct power and dither modulator control techniques can be successfully implemented into standard WDM transmission equipment, where standard continuous wave (CW) lasers are used with fixed wavelengths. However for next generation systems using optical burst switched technology there are a number of limitations to the prior art control schemes making them unusable.
For burst mode transmitters, lasers are rapidly switched on and off and can be set to any wavelength or power. Therefore the control loop must be insensitive to very fast laser power and wavelength changes. Furthermore the laser may be turned off for long periods of time; therefore the modulator bias can drift with no feedback control.
For the direct optical power feedback control scheme, a fast burst envelope detector and comparator can be used to compare the optical power to a reference power for a particular wavelength setting. However a problem with this scheme is that it relies on a complex calibration procedure that can change over time.
For standard dither control schemes, the modulator input power and wavelength must remain constant during two consecutive control steps for the optical power difference to be measured correctly. Therefore the optical power must be sampled twice for each burst; requiring very fast sampling and dithering circuits, knowledge of the start and end of each burst, and wavelength dependent burst settings. This is a complex procedure, expensive to implement, and prone to large control error discrepancies due to burst transmission effects e.g. power transients.
There is therefore a need to provide a modulator control system and method to combat the problems of temperature and aging drifts of components in an optical network and optimise transmission performance of optical bursts on multiple paths using controllable transmit parameters.